A260626 a(n) = gcd(m, 2^m-1) where m is the n-th nonprime positive integer.

1, 1, 3, 1, 1, 1, 3, 1, 1, 1, 9, 5, 7, 1, 3, 1, 1, 1, 1, 3, 1, 1, 1, 1, 9, 1, 1, 5, 21, 1, 1, 1, 3, 1, 1, 1, 1, 27, 1, 1, 1, 1, 15, 1, 7, 1, 1, 3, 1, 1, 1, 9, 1, 1, 1, 1, 3, 5, 1, 1, 21, 1, 1, 1, 1, 9, 1, 1, 1, 1, 1, 3, 1, 1, 25, 3, 1, 7, 1, 27, 11, 1, 1, 3, 1

Offset

1,3

Comments

2^m - 1 is a nonprime number if m is a nonprime number.

Links

Michel Lagneau, Table of n, a(n) for n = 1..10000

Formula

a(n) = gcd(A018252(n), 2^A018252(n)-1).

a(n) = A014491(A018252(n)). - Michel Marcus, Nov 01 2015

Maple

seq(`if`(isprime(m), NULL, igcd(m, 2^m-1)), m=1..150);

Mathematica

GCDnonPrime[n_Integer]:=GCD[2^FixedPoint[n+PrimePi@#&, n+PrimePi@n]-1, FixedPoint[n+PrimePi@#&, n+PrimePi@n]]; Array[GCDnonPrime, 120]
GCD[#, 2^#-1]&/@Select[Range[200], !PrimeQ[#]&] (* Harvey P. Dale, Aug 25 2019 *)

Prog

(PARI) for(n=1, 1e3, if(!isprime(n), print1(gcd(n, 2^n-1)", "))) \\ Altug Alkan, Nov 01 2015

Crossrefs

Cf. A014491, A018252.

Sequence in context: A060901 A351545 A087612 * A155828 A226203 A327791

Adjacent sequences: A260623 A260624 A260625 * A260627 A260628 A260629

Keyword

nonn,easy

Author

Michel Lagneau, Oct 31 2015

Status

approved